Method for fabricating expanded steel generator retaining rings



United States Patent 3,336,785 METHOD FOR FABRICATING EXPANDED STEEL GENERATOR RETAINING RINGS Samuel J. Manganello, Penn Hills Township, Allegheny County, and John E. Steiner, Churchill Borough, Pa., assignors to United States Steel Corporation, a corpo ration of Delaware No Drawing. Filed July 8, 1964, Ser. No. 381,234 2 Claims. (Cl. 72-377) This invention relates to steel generator retaining rings and, more particularly, to a method of fabricating such rings. The invention, more specifically, resides in the discovery that cold expansion of a hot-forged annular blank to final size can be effected without first machining its inner cylindrical surface to remove the layer of decarburized steel that is formed when the blank is heated to forging temperatures.

Nonmagnetic steel retaining rings are commonly used to hold in place the field windings of large electrical generators. They are made in various sizes depending on the capacity of the generator and the diameter and speed of rotation of its rotor. The rings generally vary from 20 to 65 inches in diameter as measured at the mid-point of the wall thickness, 17 to 30 inches in length, and 2 to 4 inches in wall thickness. They may weigh from 1000 to 4000 pounds. These rings are generally fabricated from austenitic manganese or manganese-nickel steel.

In fabricating retaining rings, steel of the desired composition is melted in an electric furnace, poured as a killed ingot, and forged at about 2200 F. in a forging press. An annular blank of intermediate size is then formed by punching a hole in the center of the forged ingot, which is then forged on a mandrel to a larger size and to correct any out-of-roundness. The forging is reheated, as necessary, during the hot-forging, punching, and mandrel-forging steps. After the mandrel-forging step, the forged blank is water quenched, rough machined, solution annealed at about 2000 F. for 2 to 4 hours, and water quenched. The blank is then expanded to about its final dimension, its diameter being increased approximately 20 to 40 percent. The blank is preferably cold (under 400 F.) during expansion to develop improved properties by cold-working the steel. The fully expanded ring is given a final light machining on all surfaces.

Different methods and forms of apparatus may be used for effecting the cold expansion of the forged blank to final size. The blank may be expanded in a single operation by forcing it axially over an expanding mandrel as disclosed in Patent No. 3,078,905, isssued Feb. 26, 1963, to R. R. Somers et al., or it may be expanded by placing a plurality of expanding shoes at circumferentially spaced intervals about the inner surface of the ring and then forcing a tapered drift pin through the central slot embracing the shoes. In expanding operations of this character, the expansion is effected by applying force radially outwardly against the inner cylindrical surface of the annular blank.

According to conventional practice the annular blank produced by the mandrel-forging step is rough machined as mentioned above to prepare it for the cold expansion step. This is done to remove any eccentricity of its outer cylindrical surface with respect to its inner surface so that all portions will be subjected to uniform expansion, and to square its ends especially when-the expansion is effected as described in the patent mentioned above. In addition its inner cylindrical surface is machined to remove the decarburized surface layer formed during the heating of the blank to forging temperatures. Removal of this decarburized layer at the inner surface of the blank has heretofore been considered critical and essential in order to obtain an expanded ring of good quality. Since Patented Aug. 22, 1967 this surface layer, because of its low carbon content, may consist of unstable austenite, it was believed that the poor cold-working characteristics of such a layer if not removed would cause cracking during subsequent cold expansion of the blank. In addition it was believed that minor defects in the inside surface of the blank would result in stress concentrations that would also cause cracking during subsequent expansion.

The conventional practice of rough machining the inner cylindrical surface of the forged blank requires that in selecting the size to which the blank is to be forged various tolerances must be maintained to ensure that the desired rough-machined size can be made from the rough' forging. These include a tolerance on the inside and outside-wall diameters, as well as extra length at both ends of the blank.

Contrary to the well-established practice of rough machining the forged blanks prior to expansion, we have discovered that the blanks may be expanded by any of the presently used methods while in the as-forged condition. Hence, preliminary machining may be either eliminated or drastically reduced as hereafter described.

As previously mentioned, the forged blank must be expanded by 20 to 40 percent, depending upon the composition of steel from which the blank is constructed and the cold-worked properties desired. Therefore, the wall thickness of the blank before expansion must be such that the desired degree of expansion will result by using an expanding means, such as a plug, sufiicient to produce the preselected inside diameter after expansion.

If the wall thickness necessary to achieve the required percent of expansion can be obtained during forging. then the entire rough-machining step, of both the inside and outside surfaces, may be eliminated. The expansion for most retaining-ring steels may deviate from the preselected amount of expansion by as much as 2 percent without significantly affecting the properties resulting from cold expansion. However, if the necessary tolerances cannot be met by the forging operation, then the outside surface of the blank must be preliminarily machined to obtain the required wall thickness. The more difficult machining of the inside surface is eliminated, however, no matter which condition prevails.

Since in the practice of our invention, preliminary machining of the blankone of the most costly steps in the production of retaining ringsis not essential, the blank may be forged closer to the desired wall thickness; this requires less steel to be used in the rough forging for each ring. Also, by eliminating the rough machining of the inside surface of the blank, fewer of the costly machining set-ups are required.

In the preferred practice of our invention, an annular blank is produced by piercing a forged ingot and then enlarging it by mandrel-forging as stated above to an intermediate size. These operations result in a forged blank that has concentric cylindrical inner and outer surfaces, each of which, due to heating to forging temperatures, has a surface layer of decarburized steel. After solution annealing and air cooling, lubricant is applied to its inner cylindrical surface and the blank is then expanded while cold by applying force radially outwardly against its forged inner surface to increase its diameter to the required inner diameter of the retaining ring and to impart the expansion necessary for cold-working the metal to produce the required strength characteristics. For this purpose the blank is preferably preheated to a temperature of about 350 F. and plug-expanded as described in the above-mentioned Patent No. 3,078,905. This expansion, as explained above, is effected in accordance with the principles of this invention, with the inner surface of the blank in the as-forged condition and without prior removal of the decarburized surface layer about its inner surface by a machining operation as is customary in conventional practice. Expanded rings produced in this manner exhibited no cracks and had smooth, uniform inner surfaces acceptable for final machining. While this practice dispenses with the rough machining of the inner surface previously required, it will be understood that irregularities on such surface, such as projections or cracks may be removed by grinding or chipping within the principles of this invention.

As explained above, the outer cylindrical surface of the blank need be machined only when necessary to provide for the desired percent expansion in accordance with the inside diameters of the blank and finished ring. If the blank has an outside diameter that will provide this expansion when the blank is expanded to final size, then machining of the outer surface to reduce the outer diameter of the blank is not required. It will of course be understood that machining of the inner surface is never required.

While the above describes the preferred practice of the method of this invention, it will be apparent that other 20 adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. A method of forming expanded steel generator retaining rings which comprises the successive steps of hotforging an annular blank to intermediate size with a cylindrical layer of decarburized steel about its inner surface Which is produced as the result of heating said blank to forging temperatures, and applying force radially outwardly against said decarburized layer to increase the diameter of and expand said blank to final size.

2. A method of forming expanded steel generator retaining rings which comprises the consecutive steps of working a steel blank by hot-forging to an annular shape having a cylindrical inner surface, said blank having a layer of decarburized steel produced as the result of heating to forging temperature extending radially outwardly from said surface, and applying force against said forged inner surface to enlarge the diameter of and expand said blank to its final size.

References Cited UNITED STATES PATENTS 2,828,538 4/1958 Darden 72-370 3,078,905 2/1963 Somers et al 72370 FOREIGN PATENTS 375,682 6/1932 Great Britain.

CHARLES W. LANHAM, Primary Examiner.

25 L. A. LARSON, Assistant Examiner. 

1. A METHOD OF FORMING EXPANDED STEEL GENERATOR RETAINING RINGS WHICH COMPRISES THE SUCCESSIVE STEPS OF HOTFORGING AN ANNULAR BLANK TO INTERMEDIATE SIZE WITH A CYLINDRICAL LAYER OF DECARBURIZED STEEL ABOUT ITS INNER SURFACE WHICH IS PRODUCED AS THE RESULT OF HEATING SAID BLANK TO FORGING TEMPERATURES, AND APPLYING FORCE RADIALLLY OUTWARDLY AGAINST SAID DECARBURIZED LAYER TO INCREASE THE DIAMETER OF AND EXPAND SAID BLANK TO FINAL SIZE. 